For millions of people all over the world, running is such a natural, enjoyable and even addictive form of exercise that it can be jarring to encounter photos that remind us of just how weird it is. After all, if walking is a form of “controlled falling,” as one well-known description has it, then running can perhaps be characterized as an endless series of narrowly averted catastrophes.

People run for any number of reasons. They run for their health. They run because they want to push their own physical limits. They run because they’re neurotic. They run because working out at a gym is too confining, too smelly, too boring. They run because it makes them happy (or because it makes them forget, for a while, that they’re unhappy). They run because their friends run, and the camaraderie one enjoys when running with kindred spirits is one of life’s simple, abiding pleasures.

Whatever the reasons, on National Running Day LIFE offers photos by a pioneer of stroboscopic photography, Gjon Mili, that neatly illustrate the melding of grace and power one applauds — but rarely ever really gets a chance to examine — in the very best runners. Granted, most of these pictures are of sprinters rather than distance runners, but the elemental beauty, the smooth flowing movement of the enterprise pertains to virtually all types of running.